Open AI Responses API vs. Chat Completions vs. Anthropic Messages API
Blog post from Portkey
The landscape of Large Language Model (LLM) APIs is currently diverse yet crucial for teams transitioning from prototypes to production, with the choice of API format influencing vendor flexibility, codebase complexity, and model-swapping ease. Three primary API formats are prominent: OpenAI's Chat Completions API, known for its simplicity and wide ecosystem compatibility; OpenAI's Responses API, designed for agentic workflows with built-in tools and state management; and Anthropic's Messages API, which offers Claude-native capabilities such as extended reasoning and prompt caching. Each format has distinct features suited to different use cases, from general text generation to complex reasoning and tool integration. Portkey offers a solution by supporting all three API formats, enabling seamless integration and model-switching without altering application code, while providing additional benefits like observability, load balancing, and cost tracking. This flexibility allows developers to utilize any format with any provider, facilitating an adaptable and efficient AI strategy.